We are participating in a multi-center Phase I/II trial with the Pediatric Oncology Group. The primary objective of this study is to assess the maximum tolerated dose of PSC-833 in combination with mitoxantrone and etoposide in pediatric patients with relapsed or refractory leukemia. The secondary objectives are to determine the effects of PSC-833 on mitoxantrone and etoposide pharmacokinetics. The MDR1 gene expression and P-glycoprotein function will be evaluated in patient derived leukemia cells. The GCRC will provide support with processing of blood samples obtained for patients enrolled at Stanford, in addition it will provide pharmacokinetic services for all patients enrolled in this study across the US and Canada. To date samples from six patients have analyzed in the pharmacokinetics laboratory.